1. Field of the Invention
The present invention relates to a display converter of a liquid crystal display panel used as a display unit in various kinds of data processors such as a word processor, a work station, etc. More particularly, the present invention relates to an improvement of a liquid crystal display converter for a liquid crystal display panel of a so-called two-divisional driving system in which the liquid crystal display panel is operated by dividing a display screen into a plurality of divisional screens such as two divisional screens, etc. Concretely, the present invention relates to a liquid crystal display converter in which a high speed display is performed without using any high speed frame memories so that a cost of the liquid crystal display converter can be reduced.
2. Description of the Related Art
A liquid crystal display panel as a display unit generally tends to be used in many cases in various kinds of data processors such as a word processor, a work station, a desk top publishing (DTP) device, etc. The liquid crystal display panel is called an LCD in the following description.
In this case, a liquid crystal display converter is used as a display controller for the LCD so as to cope with a situation in which no change in software is required. The liquid crystal display converter has a function for converting a display signal of the display unit such as a cathode ray tube (CRT) generally Used to a display signal for the LCD.
This general liquid crystal display converter generates data for a display by changing the display signal from the display controller in a data processor to the display signal for the LCD.
The liquid crystal display converter generally used will next be explained with reference to FIG. 1.
FIG. 1 is a functional block diagram showing one example of a main construction of the general liquid crystal display converter. In FIG. 1, reference numerals 1, 2 and 3 respectively designate a display controller, a write address generating section and an interface section for a frame memory. Reference numerals 4, 5 and 6 respectively designate an LCD control section, a read address generating section and an address generating section of the frame memory. Reference numerals 7, 8 and 9 respectively designate an interface section for an LCD, the frame memory and the LCD.
As shown in FIG. 1, the general liquid crystal display converter has one frame memory 8. Display data outputted from the display controller 1 are converted to data for the frame memory by the interface section 5 for the frame memory and are then written to this frame memory 8.
A write address at this time is generated by the write address generating section 2 by using a control signal outputted from the display controller 1. This write address is then transmitted to this frame memory 8 through the frame memory address generating section 6.
The display data to be transmitted to the LCD 9 are read out of the frame memory 8 and are converted to data for the LCD by the interface section 7 for the LCD. These converted data are transmitted to the LCD 9.
In this case, a read address is generated by the read address generating section 5 by using a control signal outputted from the LCD control section 4 for converting a synchronous signal from the display controller 1 to a control signal for the LCD. This read address is then transmitted to the frame memory 8 through the frame memory address generating section 6.
The display controller 1 is constructed as shown in FIG. 2.
FIG. 2 is a functional block diagram showing one example of a detailed construction of the display controller 1 shown in FIG. 1. In FIG. 2, reference numerals 10, 11, 12 and 13 respectively designate a central processing unit (CPU), a display circuit, a video RAM and a display unit. As shown in FIG. 2, the video RAM 12 in the display controller 1 shown in FIG. 1 holds data for a display from the central processing unit (CPU) 10. The display circuit 11 in the display controller 1 controls writing and reading operations of this video RAM 12 and generates a control signal transmitted to the display unit.
In the general liquid crystal display converter, data required for a display are outputted to the LCD 9 by the above construction and operation so that a picture image is visually displayed on the screen.
A so-called two-divisional driving system is used in the LCD. In this driving system, for example, the screen of the LCD is divided into two divisional screens composed of upper and lower divisional screens in view of the relation of afterimage characteristics, etc. when the screen is especially large-sized. The LCD is operated every divisional screen in this driving system.
If this driving system is used, it is possible to provide a screen having a high grade with reduced flicker even when the screen of the LCD is large-sized.
In this case, the frame memory 8 holds only image data corresponding to one screen in the general two-divisional driving system as already mentioned above. Therefore, in the frame memory 8, two reading operations are required with respect to one writing operation.
This relation will next be explained with reference to FIGS. 3a through 3e.
FIGS. 3a through 3e is a timing chart for explaining one example of the writing and reading operations of data with respect to the frame memory in the liquid crystal display panel of the general two-divisional driving system.
In FIGS. 3a through 3e, for example, a cycle shows an access period provided when an access operation with respect to each of addresses on the upper and lower divisional screens is performed.
In this example, cycles n-1, n and n+1 are shown.
Reading data m shows n-th data m on the lower divisional screen corresponding to n-th data n on the upper divisional screen.
Upper data of the LCD shown in FIGS. 3a through 3e are data displayed on the upper divisional screen of the two divided screens. Lower data of the LCD shown in FIGS. 3a through 3e are data displayed on the lower divisional screen of the two divided screens. FIGS. 3a through 3e shows output timings of these upper and lower data.
As shown by the timing chart of FIGS. 3a through 3e, when certain data on the screen such as n-th data are accessed, writing data (n) are written with respect to one cycle of the LCD. Thereafter, reading data (n) are read as data for the upper divisional screen of the LCD set to the upper data of the LCD in FIGS. 3a through 3e. Next, it is necessary to read reading data (m) as data for the lower divisional screen of the LCD set to the lower data of the LCD in FIGS. 3a through 3e. Accordingly, three access operations are performed with respect to the frame memory 8 to display a picture image on one screen.
If such operations are performed, it is possible to display a picture image having a high quality and no flicker even when the screen of the liquid crystal display panel (LCD) is large-sized.
For example, a known integrated circuit (IC) of an HD66840 type manufactured by Hitachi Co., Ltd. in Japan has such a function.
As mentioned above, the LCD of the two-divisional driving system is generally known and an integrated circuit (IC) for this LCD is also known.
However, a double reading operation is performed with respect to the frame memory 8 in the LCD of the two-divisional driving system. Therefore, it is necessary to terminate the reading operation every divisional screen within a time half that for a writing operation.
Accordingly, it is necessary to use a high speed frame memory and thereby the cost of the liquid crystal display converter is increased.
In a memory having a single port, it is necessary to operate this memory at a high speed. Accordingly, it is considered to use a memory having a dual port composed of a random access port and a serial access port.
The reading and writing operations can be simultaneously performed in accordance with a method for using this memory having the dual port. Accordingly, a predetermined object of the liquid crystal display converter can be achieved without using any high speed memory.
However, cost of the dual port memory is high by about several times in comparison with the single port memory at the same operating speed so that price of the liquid crystal display converter is similarly increased.